Add "Put Back" button to cancel accidental discard draws

When you accidentally click the discard pile, you can now put the card
back instead of being forced to swap. The "Put Back" button appears
only when you've drawn from the discard pile.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

client/index.html

@@ -281,6 +281,7 @@
                                             <span id="discard-content"></span>
                                         </div>
                                         <button id="discard-btn" class="btn btn-small hidden">Discard</button>
+                                        <button id="cancel-draw-btn" class="btn btn-small btn-secondary hidden">Put Back</button>
                                         <button id="skip-flip-btn" class="btn btn-small btn-secondary hidden">Skip Flip</button>
                                         <button id="knock-early-btn" class="btn btn-small btn-danger hidden">Knock!</button>
                                     </div>